In steps for semiconductor production, a treating liquid containing an acid (e.g., sulfuric acid) and hydrogen peroxide is used in silicon wafer pretreatment, etc. As this treating liquid is repeatedly used, accumulation of impurities such as silica and decomposition of the hydrogen peroxide proceed to lower the acid concentration, resulting in a decrease in treatment function. It is hence necessary to renew the treating liquid intermittently. Because of this, a waste liquid having a high acid concentration and containing residual hydrogen peroxide is discharged in a large quantity. The amount of this waste liquid is affected by the regulations of the industrial waste management law.
Consequently, it becomes important to develop a technique by which the hydrogen peroxide remaining in the waste liquid is safely decomposed to a recycling level and which enables the concentrated acid (sulfuric acid) in the waste liquid to be recovered and be recycled or sold. In cases when the hydrogen peroxide is decomposed and the remaining acid is thereafter treated by neutralization, then there are cases where the separation by floatation of the neutralization-yielded precipitate which has risen up is difficult due to the remaining hydrogen peroxide.
As methods for decomposing hydrogen peroxide, the following techniques are, for example, known.
(1) A method in which a waste liquid containing sulfuric acid and hydrogen peroxide is neutralized using an alkali agent, such as sodium hydroxide, calcium hydroxide, or ammonia, and the hydrogen peroxide is then removed.
(2) A method in which no alkali agent is used, wherein the hydrogen peroxide is decomposed with a hydrogen peroxide decomposition catalyst while supplying hydrogen to the catalyst (see patent document 1).
(3) A method in which the waste liquid having an elevated temperature is passed through a carbonaceous substance to decompose the hydrogen peroxide (see patent document 2).
(4) A method in which use is made of an activated-carbon treatment device where a liquid to be treated that contains hydrogen peroxide is caused to flow upward through an activated-carbon tower packed with granular activated carbon, from the bottom side toward the top side, thereby bringing the liquid to be treated into contact with the granular activated carbon to decompose the hydrogen peroxide, and in which the activated-carbon layer constituted of the granular activated carbon packed into the activated-carbon tower has a top-side cross-sectional area that is larger than on the bottom side (see patent document 3).